Domino diels-alder cycloadditions of arynes: New approach to elusive perylene derivatives

Article abstract of DOI:10.1002/chem.201001057

(Figure Presented) A game of dominos: Domino [4+2]/ [4+2] cycloadditions between 1,8-difurylnaphthalene and arynes provide the corresponding adducts in a highly diastereoselective manner. This new synthetic methodology enables the preparation of elusive perylene derivatives with a remarkable reduced HOMO-LUMO gap, a crucial feature for organic semiconductors.

Full text of DOI:10.1002/chem.201001057

DOI: 10.1002/chem.201001057
Domino Diels–Alder Cycloadditions of Arynes: New Approach to Elusive
Perylene Derivatives
[
a]
Alejandro Criado, Diego PeÇa,* Agustꢀn Cobas, and Enrique Guitiꢁn*
Dedicated to Professor Josꢀ Barluenga on the occasion of his 70th birthday
9736
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Chem. Eur. J. 2010, 16, 9736 – 9740

COMMUNICATION
A huge amount of effort is currently dedicated to the
design, preparation, and characterisation of new carbon-
gaps obtained for 1a, 1b, and 1c are 2.54, 2.14, and 2.63 eV,
respectively. The first value, 2.54 eV, is in the upper limit of
the range for semiconductors. Remarkably, the calculated
[1]
based electronic materials. Carbon nanotubes, graphenes,
and related species are compounds with very interesting
electronic properties. However, to exploit the potential of
these systems some limitations need to be overcome as
there are no selective syntheses of these compounds in a
pure form and their processability is hampered by high mo-
lecular mass and low solubility. On the other hand, the
chemistry of polycyclic aromatic hydrocarbons (PAHs)
offers the possibility of tailoring a variety of skeletons and
substituents, thus allowing the modulation of properties
such as molecular size, solubility, and electronic characteris-
tics. However, only a limited number of platforms based on
PAHs are currently under systematic investigation. Among
gap for naphtho
ACHTUNGTRENNUNG[ 2,3-a]perylene (1b, 2.14 eV) is slightly
lower than the gap calculated for pentacene (2.29 eV) using
[4d]
the same level of theory. These promising results prompt-
ed us to carry out the synthesis of these platforms.
Benzo[a]perylene (1a) had been prepared by Clar and co-
[7]
workers by complex and inefficient procedures. To the
best of our knowledge, the syntheses of naphtho[2,3-a]pery-
lene (1b) and phenanthro[9,10-a]perylene (1c) have not
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
been reported. We devised a new strategy for the synthesis
of compounds 1 based on the retrosynthetic analysis shown
[8]
in Scheme 1. The target molecules 1 should be available
[2,3]
them, oligoacenes have been widely studied.
In particu-
lar, pentacene derivatives are extremely relevant in this
field, with more than 1400 papers published since 2000 deal-
[4]
ing with their electronic properties. The basic pentacene
platform is a paradigmatic molecular semiconductor with a
HOMO–LUMO gap of around 2.1 eV, although it is rela-
tively unstable against photo-oxidation. The design and syn-
thesis of pentacene derivatives with a lower gap and higher
[2c,e,4d]
chemical stability is a very active field of research.
Per-
ylene derivatives have also been developed for molecular
electronics and molecular photonics, as components of
[5]
LEDs, photovoltaic cells, and other devices.
Our experience in aryne chemistry applied to the synthe-
[6]
sis of extended PAHs led us to explore the synthesis of
novel perylene derivatives with potential applications in mo-
lecular electronics. We chose perylenes 1a–c as initial goals
for our study because these compounds are relatively small
molecules that contain acene subunits, presumably available
in a reduced number of steps, with an expected HOMO–
LUMO gap near to or below 2.5 eV, a crucial feature for or-
ganic semiconductors. We decided to study the gap of 1a–c
by density functional theory (DFT) at the B3LYP 6-311+
G** level on 6-31G(d) optimised geometries. The computed
Scheme 1. Retrosynthetic analysis.
by dehydration/deoxygenation of intermediates 2, obtained
by the reaction of 1,8-difurylnaphthalene (3) with 1,2-di-
AHCTUNGTRENNUNGd ehydrobenzene (benzyne, 4a), 2,3-didehydronaphthalene
[9]
(
4b), and 9,10-didehydrophenanthrene (4c), respectively.
1
,8-Difurylnaphthalene (3) was prepared in good to excel-
lent yields by double metal-catalysed cross-coupling of 1,8-
diiodonaphthalene (6) with (1-furyl)zinc chloride (7a),
(
1-furyl)boronic acid (7b), or (1-furyl)tributyltin (7c,
Scheme 2).
Didehydroarenes 4a–c were generated from the corre-
[10]
sponding (trimethylsilyl)aryl triflates 5a–c.
Treatment of
[
a] A. Criado, Prof. Dr. D. PeÇa, Prof. Dr. A. Cobas, Prof. Dr. E. Guitiꢁn
Departamento de Quꢂmica Orgꢁnica, Facultad de Quꢂmica
Universidade de Santiago de Compostela
these triflates with CsF in the presence of 3 afforded adducts
2a–c in 99, 51, and 53% yield, respectively (Scheme 3). This
key transformation involves two tandem cascade Diels–
Alder reactions in a domino mode, with the creation of four
new CꢀC bonds and six new stereogenic centres. Presum-
ably, the intermolecular [4+2] cycloaddition of 3 with
arynes 4a–c would lead to the formation of intermediates
1
5782-Santiago de Compostela (Spain)
Fax : (+34)981591014
E-mail: diego.pena@usc.es
enrique.guitian@usc.es
[11]
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201001057.
Chem. Eur. J. 2010, 16, 9736 – 9740
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www.chemeurj.org
9737

D. PeÇa, E. Guitiꢁn et al.
6
-31G(d) calculations, which showed that the exo,exo-2a
adduct is favoured under kinetic or thermodynamic control
Table 1).
(
Table 1. Calculated relative reaction barriers and enthalpies for the intra-
molecular Diels–Alder reaction (8a to form 2a).
[
a]
¼6
[b]
[b]
2
a
DH
DH
exo,exo
13.5
18.9
34.2
29.7
ꢀ24.0
ꢀ9.8
9.8
ꢀ8.4
exo,endo
endo,endo
endo,exo
[
a] The first stereochemical prefix indicates the face of the alkene that
Scheme 2. Synthesis of 1,8-difurylnaphthalene (3); dppp=1,3-bis(diphe-
nylphoshino)propane.
reacts intramolecularly in 8a, the second one indicates the approach to
the dienophile. [b] Calculated at 298 K (kcalmol ).
ꢀ
1
The relative stereochemistry of adduct 2c was confirmed
by single-crystal X-ray diffraction studies (Figure 1). Nota-
bly, the torsion angle between the naphthalene and the
phenanthrene subunits is 57.68.
Scheme 3. Domino Diels–Alder cycloadditions.
8
a–c, which could evolve in situ by intramolecular [4+2] cy-
cloaddition to afford exo,exo-2a–c, resulting from the exo
approach of the exo face of the dienophile. Remarkably, this
is a highly stereoselective transformation: only one diaste-
reoisomer was obtained from four possible products
Figure 1. Molecular structure (ORTEP) of 2c obtained by X-ray diffrac-
tion.
[12]
(
Scheme 4). This result is in agreement with DFT B3LYP/
Treatment of adducts 2a–c with concentrated aqueous
HCl in refluxing EtOH led to perylene derivatives 1a–c in
7
2, 57, and 87% yield, respectively. UV/Vis spectroscopy
was used to explore the electronic properties of these aro-
matic platforms. In particular, the absorption of the lowest-
energy UV/Vis absorption bands of compounds 1a–c are
shown in Table 2, they exhibit a typical vibronic structure.
Optical HOMO–LUMO gaps were determined from the
onset of these bands and are in very good agreement with
the calculated gap determined by time-dependent density
[13]
functional theory (TDDFT) calculations,
1G(d) level. The differences between experimental and
at B3LYP/6-
3
TDDFT-calculated values (0.04–0.15 eV) are smaller than
those obtained by using DFT calculations (0.12–0.33 eV,
vide supra).
Scheme 4. Four possible stereoisomers of 2a.
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Domino Diels–Alder Cycloadditions of Arynes
COMMUNICATION
Table 2. Absorption and emission data for 1a–c.
[
a]
[b]
[c]
[d]
Compound
l
max abs.
Optical gap
[eV]
Calcd gap
[eV]
lmax em.
[nm]
[nm]
1
1
1
a
b
c
507
588
508
2.36
2.02
2.30
2.41
1.99
2.42
518
604
544
[
a] Lowest-energy UV/Vis absorption band from dichloromethane solu-
tions. [b] Optical HOMO–LUMO gap determined from the onset of lmax
abs. [c] Calculated HOMO–LUMO gap determined by TDDFT calcula-
tions. [d] Emission maxima from dichloromethane solutions.
Scheme 5. Photocyclodehydrogenation of pentahelicene 1c.
conditions such as strong oxidants, prolonged UV irradia-
tion, or flash vacuum pyrolysis (FVP) are normally required
to perform the cyclodehydrogenation of pentahelicene.
Therefore, the three benzene rings fused to the pentaheli-
cene moiety in compound 1c are the crucial structural motif
for this spontaneous cyclodehydrogenation to afford poly-
It is worth mentioning that compound 1b has an absorp-
tion maximum at 588 nm (Figure 2), corresponding to an op-
tical gap of 2.02 eV, that is, smaller than the gap reported
[15]
[4d]
for pentacene (lmax =582 nm, optical gap=2.08 eV),
and
an emission maximum at 604 nm. For comparison, tetracene
and perylene, the two major structural subunits present in
compound 1b, show absorption maxima in solution at 474
AHCTUNGTRENNUNG
[14]
and 436 nm, respectively.
on highly stereoselective tandem cascade domino [4+2] cy-
cloadditions involving arynes, opens a new way to access
elusive polyarenes and in particular to key molecules for the
development of new organic semiconductors. Further work
is in progress to obtain substituted derivatives with better
solubility/processability, higher kinetic stability against
photo ACHUTNRGNENUGo xidation, and a reduced gap (<2 eV).
Acknowledgements
Financial support from the Spanish Ministry of Education and Science
MEC, CTQ2007-63244) and the Xunta de Galicia (DXPCTSUG2007/
90 and PGIDIT07PXIB209139PR) is gratefully acknowledged. We
(
0
thank Prof. Dolores Pꢃrez (Universidade de Santiago de Compostela) for
helpful discussions. Generous allocation of computer time was kindly
provided by the Centro de Supercomputaciꢄn de Galicia (CESGA). A.C.
thanks the MEC for the award of an FPU fellowship.
Figure 2. Absorption (solid line) and emission (dashed line) spectra in di-
chloromethane for compound 1b.
Keywords: arenes
· arynes · cycloaddition · domino
reactions · hydrocarbons
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Chem. Eur. J. 2010, 16, 9736 – 9740
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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Received: April 21, 2010
Published online: July 14, 2010
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Chem. Eur. J. 2010, 16, 9736 – 9740